1. Field of the Invention
The present invention relates to a DC/DC converter for converting an inputted power supply voltage into a predetermined DC voltage, and more particularly, to a current-mode DC/DC converter.
2. Description of the Related Art
DC/DC converters include DC/DC converter types which include a switching element located between a power input terminal and a terminal for outputting a predetermined DC voltage, connected to a load, wherein a predetermined DC voltage is maintained through opening and closing of the switching element. Such DC/DC converters are widely used because of their small size and the high efficiency that they provide. There exists an approach for controlling the opening and closing of the switching element, involving detecting and feeding back the current flowing in a coil connected to the switching element (for instance, Japanese Patent Application Laid-open No. H11-75367, Japanese Patent Application Laid-open No. 2003-319643, and Japanese Patent Application No. 2003-111242 which corresponds to Japanese Patent Application Laid Open No. 2004-320892). A converter using that technique is called a current-mode DC/DC converter.
FIG. 5 illustrates a circuit example of a current-mode DC/DC converter. This DC/DC converter 101 includes a switching element 114 and a coil 116 which correspond to the above-described constituent elements. Through the opening and closing of the switching element 114, power from an input power supply (VCC) is supplied via a coil 116 to an output terminal OUT, connected to a load 103, so as to preserve a predetermined DC voltage at the output terminal OUT.
In addition to the switching element 114 and the coil 116, the DC/DC converter 101 includes a switching element 115 connected in series to the switching element 114, for performing an opening and closing operation alternately with the switching element 114, a coil current detection resistor 117 for detecting current flowing in the coil 116, a smoothing capacitor 118 connected to the load-side of the coil current detection resistor 117, for smoothing voltage at an output terminal OUT, a reference current value control circuit 108 for detecting voltage on the load side of the coil current detection resistor 117, and for controlling a reference current value which is a maximum current flowing in the coil 116, a clock generator 110 for generating a reference clock CLK, a feedback circuit 109 for, in synchrony with the reference clock CLK, outputting signals for the opening and closing operation, specifically, signals for closing the switching element 114 (opening the switching element 115) until the current flowing in the coil 116 exceeds the reference current value, and for opening the switching element 114 (closing the switching element 115) when the current flowing in the coil 116 exceeds the reference current value, and buffers 111, 112 provided between the feedback circuit 109 and the switching elements 114, 115.
An electrolytic capacitor, which has large-capacitance, is ordinarily used as the smoothing capacitor 118 in this DC/DC converter 101, with a view to suppressing output voltage ripple (fluctuation) and improving transient response by changing the output current.
However, an electrolytic capacitor breaks down when an opposite voltage is applied, on account of excessive noise or by being reverse-connected by mistake since it possesses polarity, and involves also smoke-generation and ignition dangers, by virtue of its internal structure. As illustrated in FIG. 7, a capacitor has, besides a nominal capacitance C, an equivalent series resistance (ESR) derived from lead wires and the internal structure of the capacitor. The large ESR value (RESR) of an electrolytic capacitor results in a large ripple voltage.
In order to solve these problems, the use of a ceramic capacitor has been proposed, since it lacks polarity, poses no smoke-generation and ignition dangers, and has a small ESR value (RESR). However, replacing the electrolytic capacitor with a ceramic capacitor in the circuit of FIG. 5 gives rise to the below-described problems of undershoot and overshoot.
Specifically, undershoot and overshoot occur ordinarily when the output current changes sharply in response to load changes, until the DC/DC converter 101 can respond to such a change through feedback. When a large-capacitance electrolytic capacitor is used in the DC/DC converter 101, undershoot and overshoot are sufficiently suppressed by the charge accumulated in the capacitor so as not to pose problems. A ceramic capacitor, however, is problematic in that its small capacitance affords insufficient undershoot and overshoot suppression, which impairs transient response. FIGS. 6(a) and (b) illustrate this phenomenon. As illustrated in the DC characteristic diagram of FIG. 6(a), the output voltage Vo in the DC/DC converter 101 is kept at the set voltage Vref whether the output current IO increases or decreases. Large undershoot and overshoot generated upon sharp changes in the output current cannot be suppressed when a ceramic capacitor is used, as illustrated in FIG. 6(b).
Since the ESR value (RESR) of a ceramic capacitor is small, moreover, the DC/DC converter 101 is problematic in that it is prone to undergo oscillation. On account of the load 103 and the smoothing capacitor 118, the DC/DC converter 101 has 1-pole, 1-zero frequency characteristics such as the characteristic curve A and characteristic curve B illustrated in FIG. 8. The pole frequency (fP) and the zero frequency (fZ) are given by the formulas below.fP=1/(2π·RO·COUT)  (1)fZ=1/(2π·RESR·COUT)  (2)
In the formulas, RO is the resistance of the load, COUT is the capacitance of the smoothing capacitor 118, and RESR is the ESR value of the smoothing capacitor 118. The X-axis in FIG. 8 is a logarithmic scale, and the fP of characteristic curve A and characteristic curve B in the figure are depicted as coinciding with each other.
The larger the frequency difference between fP and fZ is, the larger the maximum angle of phase rotation becomes. In FIG. 8, for instance, the frequency difference between fP and fZ of characteristic curve B is larger than that of characteristic curve A, and hence the maximum angle of phase rotation of the former is larger as well. A large maximum angle of phase rotation, to which further phase rotation on account of element delay, etc., in the circuits constituting the DC/DC converter 101 (for instance, the reference current value control circuit 108, the feedback circuit 109, etc.) is added, can easily give rise to oscillation. Conversely, phase rotation is less and oscillation hardly occurs as the frequency difference becomes smaller.
Values of fP=965 Hz and fZ=24.1 KHz are obtained by substituting in formulas (1) and (2) specific values (COUT=330 μF, RESR=20 mΩ) of a case where the smoothing capacitor 118 is an electrolytic capacitor, with RO=0.5Ω. That is, fZ is 25 times as large as fP. Oscillation is unlikely to occur in practice with such a fZ and fP difference. On the other hand, values of fP=3.18 KHz and fZ=318 KHz are obtained by substituting specific values (COUT=100 μF, RESR=5 mΩ) of a case where the smoothing capacitor 118 is a ceramic capacitor. The value of fZ is thus 100 times as large as fP, a considerable frequency difference likely to result in oscillation. COUT=100 μF is the maximum capacitance of a ceramic capacitor.